This invention relates to the field of isolators for use at high frequencies and more particularly to such devices having a broadband characteristic.
Present state of the art falls generally into two types of devices, terminated circulators and resonance isolators. Both types may be used to isolate one portion of a circuit from the following stages. A circulator is a device having generally three or four ports or even comprising two or more three-port devices combined. It is a non-reciprocal device in which the energy entering at a first port is transmitted to a second port with a minimum of attenuation or loss, assuming that the second port terminates in a matching impedance, whereas, energy entering at the second port is directed almost completely to a third port, at which it may be dissipated in a matching impedance. Thus, considering ports one and two, the circulator functions as an isolator. The biggest disadvantage of this type of device is that it is inherently narrowband, since the matching impedance at the third port must be reactive and, therefore, frequency dependent.
The resonance isolator as now known in the art is typically a two-port device with isolation realized by means of a gyromagnetic resonance of the ferrite material, with no exterior unilateralizing element used. Such isolators are only effective at frequencies near the resonant frequency of the material, thus are also inherently narrowband devices. Additionally, since the gyromagnetic resonance is determined to a great extent by the strength of the applied magnetic field, the operation of the device is sensitive to temperature changes which alter that magnetic field. This limits the power handling capability of the device, since the lost energy is dissipated within the ferrite material. At temperatures above the Curie point, the exchange coupling in the ferrite material disappears, leaving it simply paramagnetic.